A construction machine such as a hydraulic excavator or the like often includes a hydraulic pump driven by an engine; hydraulic actuators driven by the hydraulic fluid supplied from the hydraulic pump; and a swing structure. Such a machine now comes in hybrid form. In a hybrid construction machine, an electric motor is used to control the operation and braking of the swing structure, and the kinetic energy of the swing structure at the time of stopping its swing motion is regenerated as electric energy. The electric energy regenerated is fed to the electric motor to drive the swing structure, thereby reducing the power of the hydraulic pump (i.e., reducing the engine load). This in turn reduces the amount of fuel consumed by the engine, leading to energy saving.
JP-2011-241653-A discloses a hybrid construction machine that has both a hydraulic motor and an electric motor as swing motors to swing its swing structure (i.e., hybrid swing motion is achieved). In such a machine, the hydraulic swing motor and other hydraulic actuators (hydraulic cylinders) are typically disposed along the same hydraulic circuit, and the hydraulic fluid suctioned by a single hydraulic pump is used to drive each of those. In this respect, the above construction machine is structurally the same as conventional construction machines in which only the hydraulic motor is used to drive the swing structure.